Continuous cardiac output by impedance measurements in the heart

ABSTRACT

A diagnostic catheter for use in measuring cardiac output in the right ventricular chamber of a heart includes a catheter body having an outer periphery and a distal section terminating in a distal end and a proximal section terminating in a proximal end. A plurality of spaced electrodes are secured to the body outer periphery along the body distal section. A plurality of electrical leads extend in the catheter body from a respective one of the electrodes to the proximal end of the catheter body. An elongated rigid member is provided for stiffening a portion of the catheter body. One end of the rigid member is located adjacent a proximal most one of a plurality of electrodes. The rigid member so locates the plurality of electrodes as to space them away from endocardial tissue. The catheter is used with a cardiac output monitoring system. Signals from the catheter are acquired by a signal conditioning and catheter control unit and, are thereafter fed to a microcomputer. The catheter and the system are used in a method for determining the instantaneous volume of blood in a heart chamber.

BACKGROUND OF THE INVENTION

This invention generally relates to medical apparatus for measuringcharacteristics of a heart. More particularly, the invention relates toa balloon flotation electrode catheter which can be used withappropriate equipment to monitor cardiac outputs on a beat-by-beat basisover a prolonged period of time.

While the invention is particularly applicable to the measurement ofcardiac output in the right ventricular chamber of a human heart, itshould be appreciated that the measurement of cardiac output in anotherchamber of a heart, such as the left ventricular chamber and of anonhuman heart such as a suitable mammalian heart can also be performedby the present invention.

Several parameters are routinely monitored in patients having heartproblems or those undergoing cardiovascular surgery. These include theelectrocardiogram (ECG), the arterial blood pressure (ART), the centralvenuous pressure (CVP), the pulmonary artery pressure (PAP), and thecardiac output (CO). With the exception of cardiac output, technologynow exists which permits these time varying parameters to be monitoredcontinuously. However, all present techniques for clinically obtainingcardiac output involve indirect methods with sample intervals of severalminutes. In addition, these techniques require either the injection ofan indicator substance or the gathering of significant respiratory andblood gas patient data.

Cardiac output is generally measured in terms of liters per minute whichcorresponds to the heart's stroke volume multiplied by heart rate.Cardiac output values change depending on the activity level of thebody, the level of metabolic demand, the condition of the heart and manyother factors. During major operations, cardiac output is clinicallysignificant because it is an indicator of how well the heart itself isperforming and it demonstrates whether a sufficient supply of blood isbeing circulated to maintain metabolic demands.

One of the indirect methods of measuring cardiac output is the Fickmethod which determines such output by examining both the oxygenconsumption of the lungs and the difference between arterial and venuousoxygen concentrations. A second method involves indicator dilution.Early indicator techniques used injectates such as cardio green dyewhich was injected as a bolus into the vascular system and allowed tomix with the venuous blood. An arterial sampling through a densitometerwas then used to measure the time varying concentration levels of dye.The concentrations recorded were directly related to the flow rate ofthe dye mixed blood through the circulatory system.

The currently accepted clinical indicator method is a technique known asthermodilution. This method relies on thermal changes as a flowindicator. A bolus of cold fluid, at least 10° C. less than thepatient's core temperature, is injected into a venuous site. Aftermixing in the right ventricle, the adjacent cooled blood and fluid passa small thermistor temperature sensor which has been placed via acatheter in the patient's pulmonary artery. The time varying temperaturechanges are directly related to the flow rate of the mixed fluid throughthe right side of the heart. Since the circulatory system is a seriescircuit, the right side value is also reflective of the left sideejections. Thus, a cardiac output can be calculated from the indicatordilution curve using a known equation.

Non-invasive techniques for obtaining cardiac output have been recentlydeveloped. Echocardiographic instruments can be used to measure aorticsizes and ventricular volumes at specific times during the cardiaccycle. Stroke volumes can then be derived from this information. In thisconnection, flow doppler instruments have been developed to measureblood velocity via external probes which are placed on the skin of thepatient and aimed at a major arterial vessel such as the ascending ordescending aorta. Cardiac output is then derived by estimating thevessel diameter in determining blood flow. Further calculations canconvert the flow determinations to cardiac output by multiplying theheart rate and the flow per beat. Also, instruments which attempt tomeasure transthoracic impedance have also been developed in an attemptto determine non-invasive cardiac output. Finally, a non-invasivetechnique known as the pulse wave contour technique has been developedwhich makes use of the concept that the area under the arterial waveformcurve is related to the stroke volume and the aortic compliance.

Each of the above recited methods has deficiencies which greatly limiteither its use and/or functionality for clinical applications,especially during surgery. The Fick principle requires special equipmentand careful attention in collecting the required samples and presenttechnology does not allow all of the required patient data to becontinuously monitored and analyzed. Non-invasive methods have alsodemonstrated severe limitations with regard to the size and expense ofequipment, the requirement for highly trained personnel and may lead toinaccurate information in patients with cardiac diseases. Finally, thethermodilution technique is not capable of providing real time data on abeat-by-beat basis.

It would be very desirable to provide the clinician with the ability toevaluate cardiac function in certain circumstances, such as withcritically ill patients or during surgery, on a continual basis sinceall other hemodynamic information except cardiac output is currentlygathered on a beat-to-beat basis. By obtaining beat-to-beat cardiacoutput, a hemodynamic assessment of the patient could be performedcontinuously by the attending staff.

Accordingly, it has been considered desirable to develop a new andimproved catheter for measuring cardiac output together with a methodfor determining the instantaneous volume of blood in a chamber of aheart and a cardiac output monitoring system with which the catheter canbe used which would overcome the foregoing difficulties and others whileproviding better and more advantageous overall results.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a new and improved diagnosticcatheter is provided for measuring cardiac output in the rightventricular chamber.

More particularly in accordance with this aspect of the invention, thecatheter comprises an elongated multilumen flexible member having adistal end and a proximal end. A first lumen extends the entire lengthof the member and terminates in a distal port. A second port extendsthrough the side wall of the member at a location immediately proximateof the distal end of the flexible member and a second lumen extends fromthe proximal end of the member to the second port. An expandable sleevesurrounds the member and spans the second port. The sleeve is inflatableby a fluid introduced into the proximal end of the second lumen. Aplurality of ring electrodes are secured to the outer surface of themember at a predetermined axial spacing. The electrodes include a distalring electrode located at a first predetermined distance proximal of thedistal end of the flexible member and a proximal ring electrode locateda second predetermined distance greater than the first predetermineddistance from the distal end of the flexible member. The electrodesfurther include a plurality of intermediate ring electrodes disposedbetween the distal ring electrode and the proximal ring electrode. Aplurality of electrical conductors extend longitudinally through a thirdlumen in the flexible member from the proximal end of the flexiblemember and are individually connected to separate ones of a plurality ofring electrodes. A first stiffening member is disposed in a fourth lumenin the flexible member and extends from a third predetermined distanceto a fourth predetermined distance.

According to another aspect of the invention, a catheter is provided formeasuring cardiac output.

More particularly in accordance with this aspect of the invention, thecatheter comprises a catheter body having an outer periphery and adistal section terminating in a distal end and a proximal sectionterminating in a proximal end. A plurality of spaced electrodes aresecured to the body outer periphery along the body distal section. Aplurality of electrical leads are provided each one of which extends inthe catheter body from a respective one of the electrodes to theproximal end of the catheter body. An elongated rigid means is providedfor stiffening a portion of the catheter body. One end of the rigidmeans is located adjacent a proximal most one of the plurality ofelectrodes. The rigid means so locates the plurality of electrodes as tospace them away from endocardial tissue.

In accordance with still another aspect of the invention, a catheter isprovided for measuring cardiac output.

More particularly in accordance with this aspect of the invention, thecatheter comprises an elongated flexible multi-lumen catheter bodyhaving an outer periphery and a distal section terminating in a distalend and a proximal section terminating in a proximal end. A balloon isattached to the distal end of the body. A first lumen extends the entirelength of the catheter body and terminates in a first port whichcommunicates with an interior surface of the balloon. A plurality ofspaced electrodes are secured to the body outer periphery along the bodydistal section proximal of the balloon. A second lumen extends from adistal most one of the plurality of spaced electrodes to the proximalend of the body. A plurality of electrical leads are provided each oneof which extends through the second lumen from a respective one of theelectrodes to the proximal end of the catheter body. A third lumen isprovided which extends longitudinally in the catheter body from theproximal end to a port which is intermediate to the plurality of spacedelectrodes.

According to a further aspect of the invention, a method is provided fordetermining the instantaneous volume of blood in a chamber of an animalheart.

More particularly, the method comprises the steps of inserting anelongated tubular catheter percutaneously into the heart chamber. Thecatheter has a plurality of longitudinally spaced electrodes on thesurface thereof which electrodes are individually connected to acorresponding plurality of terminals at the proximal end of the catheterby conductors passing through the tubular catheter. The longitudinalspacing of the electrodes are such that a distal electrode and aproximal electrode are located at the pulmonic valve and the tricuspidvalve of the heart, respectively. The distal electrode and the proximalelectrode are driven with a constant current source. The potentialsignal developed between pairs of sensing electrodes locatedintermediate the pair of driving electrodes and attributable to theapplication of the driving constant current source to the pair ofdriving electrodes is selectively and sequentially detected. Thepotentials are proportional to the instantaneous impedance of the mediumexisting between the selected pairs of intermediate sensing electrodes.The detected potential signals are then converted to digital quantities.The digital quantities are applied to a programmed digital computingdevice. A single corrected instantaneous impedance value is generatedfor each of the intermediate sensing electrodes determined to lie withinthe ventricle. The impedance value detected is due to the application ofthe constant current source to the pair of driving electrodes. A singlecorrected instantaneous impedance value is calculated for a ventricularsegment volume for each pair of the sensing electrodes. The segmentvolumes for each pair of sensing electrodes are summed to produce thetotal instantaneous ventricular volume.

According to a further aspect of the invention, an apparatus is providedfor measuring the instantaneous volume of blood in a chamber of a heart.

More particularly in accordance with this aspect of the invention, theapparatus comprises an elongated tubular intravascular catheter having aproximal end and a distal end with a pair of drive electrodes attachedto the exterior surface thereof and spaced apart from one another by apredetermined distance D1 which is less than the length dimension of acatheter section that is held in the chamber. A plurality of pairs ofsense electrodes are attached to the surface of the catheter andlongitudinally spaced therealong between the drive electrodes. The pairof drive electrodes and the plurality of sense electrodes areelectrically coupled individually to a terminal at the proximal end ofthe catheter. A constant current source of a frequency F₁ is providedtogether with a switching means which is joined to the terminals forcoupling the constant current source to a selected pair of driveelectrodes. A signal detector means is connectable through the switchingmeans to predetermined pairs of the plurality of pairs of senseelectrodes for producing signal waves corresponding to the impedance ofthe medium present between a sense electrode pair selected by theswitching means attributable to the constant current source. A means isoperatively coupled to the signal detector means for sampling the signalwaves at a predetermined rate and converting the signal waves to digitalvalues representative of impedance values. A computing means is coupledto receive the digital values. The computing means is programmed tocompute the volume of the segments between selected pairs of senseelectrodes using the formula Volume=(i_(c) ×ρ×L²)/V_(EE) where i_(c) isa known constant current source, ρ is the resistivity of the medium, Lis the distance between electrodes and V_(EE) is the measured end to endvoltage.

According to another aspect of the invention, a continuous cardiacoutput monitoring system is provided.

In accordance with this aspect of the invention, an elongated tubularintravascular catheter is provided which is adapted for insertion into apatient's heart. The catheter includes a plurality of spaced electrodespositioned on a periphery of the catheter. A distal most one and aproximal most one of the electrodes are configured as drive electrodesand the remaining electrodes are configured as sense electrodes. Each ofthe electrodes is connected to a terminal located at a proximal end ofthe catheter. A signal conditioning and control unit is provided whichis in electrical contact with the catheter through the catheterterminal. The unit comprises a constant current source, a selector meansfor coupling the constant current source to drive electrodes and asignal processing means for processing a signal received by the unit. Acomputing means is electrically connected to the unit for convertingsignal waves from the unit to digital values and then computing a strokevolume of the heart.

According to still another aspect of the invention, a cardiac outputmonitoring system is provided.

More particularly in accordance with this aspect of the invention, thesystem comprises a first signal means for sending analog data related toa stroke volume in a right ventricle of a patient's heart. A signalprocessing means is provided for processing the analog data from thefirst signal means into processed analog data. A computing means isprovided for converting the processed analog data from the signalprocessing means to digital values and thereafter computing the strokevolume of the patients heart.

One advantage of the present invention is the provision of a new andimproved catheter for use in monitoring stroke volume.

Another advantage of the present invention is the provision of a methodand apparatus for measuring stroke volume and cardiac output with anaccuracy greater than has heretofore been possible using known prior arttechniques.

Still another advantage of the present invention is the provision of amethod and apparatus for measuring stroke volume and cardiac output on abeat-to-beat basis in a continuous manner.

Yet another advantage of the present invention is the provision of acatheter which, together with apparatus for measuring stroke volumefacilitates the measurement of cardiac output on a beat-to-beat basis.The catheter can also be used in ventricular pacing and the diagnosis ofcomplex arrhythmias.

Still yet another advantage of the present invention is the provision ofa balloon catheter having a series of axially aligned electrodesextending over a predetermined length proximally of the balloon suchthat when the balloon is guided into the pulmonary outflow tract of theheart, the portion of the catheter bearing the electrodes extendsbetween the tricuspid valve and the pulmonary valve of a right ventricleof the heart.

A further advantage of the present invention is the provision of a flowdirected catheter having a stiffening member contained in a lumenthereof for causing the catheter to assume the correct orientation in aright ventricle of the heart.

A still further advantage of the present invention is the provision of amethod and apparatus for measuring ventricular volume of a heart whereinthe catheter is capable of conducting stroke volume measurements usingtwo different techniques so that a comparison or a calibration can beperformed.

A yet further advantage of the present invention is the provision of aventricular volume measuring system including a catheter havingelectrodes, a signal conditioning and catheter control unit and amicrocomputer. The system allows any electrode pair to be selected foruse as either sensing electrodes or drive electrodes as desired and theelectrodes can be scanned to determine catheter position.

Still other benefits and advantages of the invention will becomeapparent to those skilled in the art upon reading and understanding ofthe following detailed specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangementsof parts, a preferred embodiment of which will be described in detail inthis specification and illustrated in the accompanying drawings whichform a part hereof and wherein:

FIG. 1 is a plan view of a catheter according to the preferredembodiment of the present invention;

FIG. 2 is an enlarged cross-sectional view along line 2--2 of thecatheter of FIG. 1;

FIG. 3 is a block diagram of a continuous cardiac output measuringsystem according to the present invention;

FIG. 4 is a front elevational view of a signal conditioning and controlunit housing of the system of FIG. 3 according to the present invention;

FIG. 5 is a block diagram of the electronic modules within the signalconditioning and control unit of FIG. 4;

FIG. 6 is a block diagram at the input isolation unit of FIG. 5.

FIG. 7 is a block diagram of the signal processing unit of FIG. 5.

FIG. 8 is a block diagram of the interface/ oscillator unit of FIG. 5.

FIG. 9 is a block diagram of the major sections of the signalconditioning and catheter control unit of FIG. 4;

FIG. 10 is a block diagram of a microcomputer of the system of FIG. 3;

FIG. 11 is a block diagram of the three primary software modulesutilized in the computer of FIG. 10;

FIG. 12 is a flow diagram of the software routines in module 1 of themodules illustrated in FIG. 11;

FIG. 13 is a flow diagram of the software routines in module 2 of themodules illustrated in FIG. 11;

FIG. 14 is a flow diagram of the software routines in module 3 of themodules illustrated in FIG. 11;

FIG. 15 is a sectional view of a heart showing the catheter of FIG. 1inserted in the right ventricle;

FIG. 16 is a perspective view of a removed section of the rightventricle of the heart of FIG. 15;

FIGS. 17A-17I are schematics of the actual circuitry in which anembodiment of the subject system is presented; and

FIGS. 18A-18II is a listing of the software modules with which thesubject system operates.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, wherein the showings are for purposes ofillustrating a preferred embodiment of this invention only and not forpurposes of limiting same, FIG. 1 shows the subject new diagnosticcatheter A which is adapted to be positioned in a heart B as isillustrated in FIG. 15 and is adapted to convey information to thecontinuous cardiac output measuring system C illustrated in FIG. 3.While the catheter will be described for use in monitoring cardiacoutput in the right ventricle of a human heart, it should be appreciatedthat the catheter can be used for monitoring cardiac output elsewhere inthe heart, such as in the left ventricle, and can also be used tomonitor cardiac output in hearts other than human hearts, such assuitable mammalian hearts and others.

More specifically, the catheter A is a balloon flotation catheter of thetype known as a Swan-Ganz catheter. The catheter A comprises anelongated tubular member 10 which can be approximately 110 cm long ifdesired and which can be made from a plasticized PVC extrusion, ifdesired. The member 10 is extruded so as to have a predetermined outerdiameter which, for purposes of illustration only, may be about a French7.5 diameter (2.475 mm) and which is preferably formed from siliconerubber, polyurethane or some other suitable plastic that tends to benon-thrombogenic. It should be appreciated, however, that the tubularmember could have a diameter between about French 4 (1.32 mm), forpediatric applications, and French 8 (2.64 mm). The tubular member 10includes a distal section 12 having a distal end 14 and a proximalsection 16 having a proximal end 18 which terminates in a pigtail sheath20.

Extending from the pigtail sheath are a first inlet tube 22, a secondinlet tube 24, a third inlet tube 26, and a fourth inlet tube 28. Alsoextending from the sheath is a first electrical conduit 30 and a secondelectrical conduit 32. Secured on a free end of the first inlet tube 22is a connector terminal 34. Similarly secured on the free ends of thesecond and third inlet tubes 24 and 26 are suitable second and thirdconnector terminals 36 and 38. To a free end of the fourth inlet tube 28is secured a fluid connector terminal 40 known as a Luer valve. A firstelectrical terminal 42, which is for the thermistor and can be a threepin Edwards type connector if desired, is connected to a free end of thefirst electrical conduit 30. Similarly, secured to the free end of thesecond electrical conduit 32 is a suitable second electrical terminal44, which is for the electrodes and can include ten pins, if desired.

The distal end 14 of the catheter is provided with a first outlet port50 which is in fluid communication with the first inlet tube 22 througha first or distal lumen 52 as shown in the cross-sectional view of FIG.2. Similarly, second and third outlet ports 54, 56 are in fluidcommunication with a respective one of the second and third inlet tubes24, 26, through suitable lumens only one of which, 58, is illustrated inFIG. 2 since the port 56 can terminate the other lumen before thecross-sectional view of FIG. 2. A balloon section 60 is in fluidcommunication with the fourth inlet tube 28 through a third lumen 62 asis illustrated in FIG. 2.

Formed through the side wall of the tubular member 10 in the zonespanned by the balloon 60, is a port, not visible in FIG. 1, whichcommunicates with the third lumen 62. Thus, when fluid under pressure isintroduced through the open fluid terminal 40, it flows through thelumen 62 and out the mentioned port so as to inflate the balloon. Bythen closing the valve 40, the balloon can be retained in its inflatedstate.

Secured on an outer periphery of the tubular member 10 are a pluralityof spaced ring type surface electrodes 70, which can be made fromElgiloy. The electrodes are spaced apart by approximately 0.8 to 1.0 cmand can be approximately 2 mm wide. The most proximal electrode isidentified by the numeral 70P and the most distal electrode isidentified by the numeral 70D. Preferably, ten electrodes are providedwith each of the electrodes being connected to a separate insulatedconductor 72 which is positioned in a fourth or electrical lumen 74 asis illustrated in FIG. 2. If desired, the distal-most electrode 70D canbe located approximately 9 cm from the distal end of the catheter withthe proximal-most electrode being located approximately 16.4 cm from thecatheter distal end, when the electrodes are spaced apart by 0.8 cm.Such an electrode spacing may be advantageous for patients with smallventricles. The conductors 72 extend in the fourth lumen proximally tothe second electrical terminal 44 and terminate in individual connectorpins 76 contained in the terminal or housing 44. The terminal is adaptedto be connected to a control unit as described hereinbelow.

Located on the tubular member 10 is a port 80 adjacent the balloonsection 60 for holding a conventional thermistor element 82 which isnormalized for blood temperature measurement and is disposed within thetubular member. As is well known in the art, a suitable plastic such aspolyurethane having good heat conducting properties covers thethermistor in the port 80 in order to prevent the ingress of blood andother body fluids. The thermistor 82 is in electrical contact with thethermistor terminal 42 through a suitable insulated conductor 84 (FIG.2) which for the sake of convenience, can also extend through the fourthlumen 74 if desired.

As illustrated in FIG. 2, a metallic stiffening member or stylet 90 issuitably disposed in a lumen 92 proximally of the proximal mostelectrode 70P. If desired, the lumen 92 can be a continuation of thelumen which leads also to the third port or proximal port 56. In orderto prevent fluid from flowing further down this lumen, a suitableadhesive plug (not visible) is suitably injected into the lumen at alocation distal of the port 56, as is well known in the art.

As is evident from FIG. 2, the tubular member can be a five lumencatheter. However, it should be recognized that the member could also beprovided with six or more lumens if that was considered desirable ornecessary.

The stiffening stylet 90 can comprise a suitable stainless steel wirewhich is encapsulated in an insulating material such as nylon. In orderto give the wire considerable stiffness, it can be made out of asuitable conventional spring wire if desired. The stylet 90 can bepositioned immediately proximally of the proximal most electrode 70P andcan extend approximately 10 cm proximally therefrom as is illustrated inFIG. 1. During insertion, the stiffening stylet 90 aids in the properpositioning of the catheter to locate the electrodes away from the heartchamber walls thereby allowing the catheter to be placed in a positionwhich permits impedance measurements.

While the stylet 90 is shown in FIG. 1 as being substantially straight,it should be appreciated that curved, bent, or looped stylets mightprove advantageous for certain catheter uses as well. The stylet couldbe fixed or adjustable as may be required. While the stylet has beenillustrated as being made of stainless steel, other types of material,such as for example fiber-reinforced composites may be used instead.

The first lumen 52 which terminates in the first port 50 at the tip ofthe catheter is useful for monitoring blood pressures during insertionof the catheter. Additionally, blood samples can also be drawn from thefirst port 50. The third port or proximal port 56 with which the lumen92 can communicate as explained above, can terminate approximately 30 cmfrom the distal end of the catheter. When the catheter is correctlyinserted in the heart, the port 56 will be located in the right atrium.This port can be used to monitor central venuous pressures and can alsobe employed as an injection site for fluids and medications. Bloodsamples can also be obtained through this port.

As mentioned previously, it is advantageous to provide a second port 54which is located between the series of spaced electrodes 70. The lumen58 communicating with port 54 can terminate at approximately the 15 cmmark as measured from the distal end locating the port between theeighth and ninth electrodes 70. The port 54 can be used for measuringright ventricular pressures as well as determining catheter location byexamining the changes in the pressure wave-form as the port passesthrough the tricuspid valve and into the right ventricle.

In another embodiment of the invention, ten electrodes can be spacedapart at 1 cm intervals beginning 9 cm from the distal tip of thecatheter and terminating 20 cm from the distal tip. A calibratedthermistor bead can be located approximately 4 cm from the distal tip.The catheter can have a balloon of approximately 1.5 cc volume locatedbetween the thermistor and the distal tip. A stiffening or stabilizingstylet 10 cm in length can be provided in the catheter between 20 cm and30 cm from the distal tip of the catheter, that is proximally from theproximal-most electrode. The stylet can be made of stainless steel whichis encapsulated nylon.

This catheter can include four lumens, namely, a proximal lumen whichterminates 30 cm from the distal end of the catheter for monitoringcentral venuous pressures, injecting fluids and medications and drawingblood samples; an electrical lumen which contains the leads for thethermistor and each of the ten electrodes; a balloon lumen which is usedto control the inflation and deflation of the balloon; and a distallumen which terminates at the tip of the catheter, for monitoring bloodpressures and drawing blood samples.

With reference now to FIG. 15, the catheter A can, if desired, beinserted via the superior vena cava. The site of entry can be aninternal jugular, subclavian or antecubital vein. Insertion and finalcatheter positioning are guided by pressure waveforms and EKG signalsobtained from the catheter. The methods employed for introducing thecatheter are identical to those used for the insertion of a conventionalSwan-Ganz catheter, and, accordingly, no further description of them isconsidered necessary. Once the distal tip of the catheter has beenrouted through a right atrium 100 of the heart B, and a tricuspid valve102 thereof and into the right ventricle 104, and inflating fluid isapplied under pressure to the balloon lumen 62 to inflate the balloon60. As blood is pumped from the right ventricle, the balloon 60 tends tobe carried by blood flow through the pulmonary valve 106 and into thepulmonary outflow tract. Once the tip of the catheter has been locatedin the pulmonary artery, it is advanced until a wedge condition exists,i.e., the inflated balloon lodges in a branch of the pulmonary artery108.

When correctly located, the proximal electrode 70P is located adjacentthe tricuspid valve 102 and the distal electrode 70D is located at theentrance to the pulmonary outflow tract and preferably adjacent thepulmonic valve 106. Once the catheter is installed, stroke volumemeasurements can be taken using the techniques set out hereinbelow.

One advantage of the pentamerous lumen embodiment of the inventionillustrated in FIG. 1, is that the port 54 can be used to injectmedications directly into the cardiovascular system even when bloodpressure measurements are being taken through the ports 50 and 56. Also,the port 54 will be positioned in the right ventricle (as shown in FIG.15) which is advantageous for obtaining a good mixing of the medicationwith the blood.

On the other hand, the port 56 can also be used to inject medication.This port, since it will be positioned in the right atrium (see FIG. 15)will also assure a good mixing of medication with the blood.

Turning now to FIG. 3, a block diagram of a continuous cardiac outputmeasuring system C of the present invention will be described. Theentire monitoring system C is contained in the portable cart D. Themonitoring system C receives electrical power from a power sourceconnection 110. Power entering through connection 110 passes through anisolation transformer 112, and then to a power distribution network 114which functions to condition power to appropriate levels and distributeit throughout the system. The power isolation transformer 112 functionsto provide a level of patient safety for the equipment when operating ina critical environment.

Signals received from the multi-electrode catheter A into the continuousmonitoring system C are acquired by a signal conditioning and cathetercontrol unit 118 ("SCCCU"), the user interface of which is illustratedmore fully by FIG. 4. The SCCCU 118 provides a user interface to controloperation parameters of the system. Included is user selected autoposition control; pacer balancing controls; input channel gain select;electronic filtration; position control; signal gain; and a master powercontrol.

It will be recalled that analog signals are received by the continuousmonitoring system C. Signals received by the unit 118 are passed througha gain select 120 which functions to isolate a desired signal level.Analog outputs from the gain select 120 are fed to a four channel analogrecorder 122, which in turn interfaces a patient monitor through aninterface adapter 124. Analog signals from the gain select 120 are alsofed to a microcomputer 130 via an analog to digital ("A to D") interface132. In this fashion, a digital signal representative of the analogvalues obtained from the multi-catheter electrode A is obtained for usein the microcomputer 130 which, in the preferred embodiment, is digital.The microcomputer 130 will be described more fully in conjunction withFIG. 10, below.

The microcomputer 130 is also in data communication with a hard-copydata recorder illustrated by printer 134. The microcomputer 130 is alsosimilarly in data communication with an external display such as thatillustrated by display screen 136 which is suitably comprised of aconventional cathode ray tube ("CRT") display. The microcomputer 130 isalso shown as including a contiguous CRT monitor 138, a data entrydevice such as key board 140, and a mass storage medium 142 which isillustrated as a pair of disk drives 142a and 142b. The mass storagemedium 142 is suitably comprised of a hard disk, a floppy disk, aCD-MEMORY (compact disk memory), or the like, or any combinationthereof. A data port 146, which is suitably comprised of a parallel portor a serial port, provides a means for communicating data to an exteriorof the microcomputer 130. As illustrated, the data port 146 communicatesdata back to the signal conditioning and catheter control unit 118 in afeed-back manner.

Turning now to FIG. 5, a block diagram of the signal conditioning andcatheter control unit 118 is presented. Power is received into the SCCCUunit 118 via interconnect 150 which is in turn connected to the powerdistribution network 114 (FIG. 3). The power network interfaces circuitbreakers 152 and a power transformer 154, which steps down the voltagetherethrough to suitable levels for operation of the remainingcircuitry. The control unit 118 includes primary circuit modulescomprising an input isolation unit 158, a signal processing unit 160,and interface/oscillator/filter unit 162, a power supply filter unit164, and a power supply unit 166. All devices are interfaced via acommon signal bus 170. The signal bus 170 also interfaces the controlpanel of FIG. 4.

The power supply unit 166 receives power from the power transformer 154,stepping it down to appropriate values for use throughout the controlunit 118. The voltage levels obtained from the power supply unit 166 arefiltered, prior to distribution to the remaining circuitry of the unit,by power supply filter unit 164.

Use of the common connector bus 170 provides a means by which any or allof the above units may be implemented by "plug-in" modules whichfacilitates selective replacement, enhancement, or modification.Implementation of this bus structure also provides for minimization ofnoise problems.

The signal bus 170 also interconnects the multielectrode catheter A viaan input protection circuit 174. The input protection circuit 174isolates the signal bus 170, and accordingly the remaining componentsinterfaced thereto, from voltage levels which may otherwise damagecircuitry within the control unit 118.

Turning to FIG. 6, the input isolation unit 158 will be described indetail. The input isolation unit 158 contains circuitry which providesfive channels of input isolation, a constant current source, electrodeselect control, and ground isolation. All signals, as well as powerentering or leaving this module, are isolated via opto-isolators ortransformer coupling. Blocks within the dash line of FIG. 6 areisolated. Blocks through which the dash line passes are providing theisolation. Block 180 illustrates a series of six high impedance voltagefollower amplifiers which function as buffers between themulti-electrode catheter and the remaining circuitry. Outputs from theinput buffers 180 are in turn fed to a series of five channels ofdifferential input amplifiers illustrated by block 182. Signalsresultant from the amplifiers 182 are in turn fed outward, again throughthe signal bus 170 (FIG. 5) via a group of opto-isolators 184. Theopto-isolators isolate the signals passed therethrough from the nextstage via optic coupling. This circuitry is powered via a transformercoupled with non-earth ground references. This forms the iso-power input190. The iso-power used is referenced to a potential known as isolatedground. Isolated ground is not tied to earth ground. This featureprovides a level of patient isolation preventing currents which areflowing due to ground reference potentials from passing into thiscircuitry. Each optical coupler provides a signal isolation well aboveseveral thousand volts.

Turning to FIG. 7, fabrication of the signal processing unit 160 of FIG.5 will be described. The signal processing unit 160 contains circuitrywhich provides AC buffering, bandpass filtering, amplitude demodulation,signal smoothing, signal inversion, and waveform isolation. FIG. 7depicts a signal flow and control related to this module. The board isconnected, via the signal bus 170, to two external devices. Theseexternal devices include a circulating fan (not shown) via aninterconnect 192, and an offset control (not shown) via an offsetcontrol interconnect 194. Unlike the circuitry of FIG. 5A the signalprocessing unit 160 includes no circuitry which uses isolated power.Outputs of the optoisolators 184 from FIG. 6 form an input to ACbuffering circuitry 200. This circuitry buffers analogously to thebuffers 180 of FIG. 6. Waveforms from the buffer circuitry 200 arepassed to a series of five 2 Khz bandpass filters 202. Use ofmulti-feedback active bandpass filters permits modulated signals of upto 40 Hertz to pass with minimal effect. The filters are implementedgiven that the modulated impedance signals contain frequencies of up to40 Hertz. The bandpass filters effectively block undesirable physiologicsignals such as electrical signals generated by the contractions of theheart.

After the bandpass filters 202, the signals are passed into fivechannels of amplitude demodulation present in demodulation circuitry204. In this circuitry, each channel of impedance waveforms is amplitudedemodulated by an absolute value and wave smoothing circuit comprised ofa pair of operational amplifiers. The output waveforms from each ofthese stages is a demodulated signal with some carrier frequency noise.The signals are next passed to a low pass filter 206 to further reducecarrier frequency noise. The low pass filters 206 are suitably comprisedof Butterworth-type filters with a maximally flat frequency response.Attenuation is suitably 12 dB at twice the cutoff frequency of 100Hertz. Signals after the 100 Hertz low-pass filters represent real timedynamic impedance waveforms for each of five selected pairs ofelectrodes from the catheter A (FIG. 1).

Signals are next fed to a signal inversion circuit 210. In this stage,each impedance value is inverted to form an admittance value. Theadmittance signal level is desired as it forms a signal from whichvolume to be measured is directly proportional. The signal inversioncircuit functions by implementation of an analog divider chip. Impedancesignals are used as denominator values, while an adjustable but constantvoltage is used for a numerator. With these two analog voltages, a realtime quotient (admittance) is developed for each channel which isinversely proportional measuring impedance values. One of two resistivevoltage dividers can be selected, via the front panel control of FIG. 4,to determine the constant numerator voltage.

Output from channels are selected from the signals from the signalinversion circuit 210 by channel selector circuit 212. Such selection isaccomplished by means of a signal on waveform select control lines whichare obtained as described further below. These control lines determinewhether a particular channel is selected. In the event a channel is notselected, no connection is made to further stages, thereby leaving thecircuitry open or at a high impedance state. Accordingly, the channelselector circuitry 212 functions analogously to a tri-state device.Outputs from the channel selector circuit 212 form inputs to a waveformsumming amplifier 216. The summing amplifier 216 generates a compositewaveform, which in turn forms an input to an offset adjust buffer 218.In the offset adding circuit 218, an externally selected DC level adds avalue to the composite admittance waveform resultant from the summingamplifier 216. This functions to "window" the waveform within the rangeof an analog to digital convertor which is implemented in the computer132 (FIG. 3).

Turning now to FIG. 8, fabrication of the interface/isolator/filter unit162 will be described. The interface/isolator/filter unit 162 providesinterface buffering, latching control, constant current sourcedevelopment, control level shifting, waveform filtering, pacemakerrejection, and auto positioning. The composite admittance waveformdeveloped by the signal processing unit 160 of FIG. 7 is routed viainterconnect 224 to circuitry which accomplishes the above-statedoperations. The interconnect 224 receives its signal from an output 220of the waveform summing amplifier 216 to FIG. 7. The composite waveforminput obtained from interconnect 224 enters a signal inverting stage 226which is comprised of an inverting buffer with unity gain. The outputfrom the inverter 226 forms an input for a filter selector switch(single pole, double throw) 228 on the panel of FIG. 4. The selectorswitch permits operator selection of an inline 40 Hertz filter or astraight-through non-filtered waveform.

The output from the signal inverting stage 226 also forms an input to alow-pass filter 230. An output of the 40 Hertz low-pass filter isselectively fed to the filter selector 228 through a pacer reject module232 prior to being additionally fed to the filter selector 228.

The pacer reject module is implemented in conjunction with the signalplaced on pacer reject line 234 which controls a selector switch 236. Asignal on the pacer reject line 234 is user selected by the controlpanel of FIG. 4. The pacer reject circuitry is comprised of a slew ratelimiter fabricated from a diode bridge circuit. The circuitry functionsby saturating if a feed-back path coupled to an RC network is not at thesame voltage as its input. A state of the feed-back path is determinedby the slew rate of the input signals. For dynamic values greater than50 Hertz, the output of the amplifier saturates. Saturation may bepositive or negative depending on a direction of a detected signalspike. Upon activation, the circuit deselects the actual wave-form andconnects the output to a high impedance source. This essentially forcesthe circuit to behave as a sample and hold circuit, thus locking outspike potentials from the pacer and forcing the circuit to remain at itslast detected value.

The operator selected signal from filter selector 228 is passed to a lowpass filter 242 and a filter selector 246. The filter selector 246functions to selectively pass the output of the filter selector 228through the 10 Hertz low-pass filter 242 prior to passing it to a gainselect potentiometer 248 which is also found in FIG. 4.

The output of gain potentiometer 248 is selectively placed, via selectorswitch 250, through an auto-position circuit 252, prior to being fed toa final output driver 254. The auto-position circuit 252 inverts thepolarity of the signal.

Turning to the top portion of FIG. 8, parallel data from themicrocomputer 130 is input to the interface/oscillator/filter unit 162via a data bus 260. Data enters control buffers 262 and 264. The buffer262 holds admittance select-channel data with the buffer 264 holdingelectrode select-line information. The lines 266 form the electrodeselect control lines which are coupled directly to the catheter A.

Data from the buffer 262 is also fed to a control level translator 270to provide for control within the interface/oscillator/filter unit 162.These signals are latched by control signal latch 272, and translated bycontrol level translator 274 which converts them to voltage levelssuitable for a logic control external of the unit 162. The voltageslevels are made available on channel select control lines 276.

Latch detect circuitry 280 triggers a control pulse at its output inresponse to a low to high transition on its input port, which is derivedfrom an output of the control data buffer 262. In response to an outputof latch detect 280, data from the select-lines control buffer 264 ismade available at lines 282.

A sine wave generator 286 has its frequency and symmetry adjusted bycontrols 288 and 290, respectively. The sine wave generated by thiscircuit is routed through an offset adjust 292 and a gain adjustpotentiometer 294. These components form a constant current source foruse elsewhere in the circuit which also includes an isolator.

Turning to FIG. 9, a summary of the interaction of all hardwarecomponents illustrated in conjunction with FIGS. 5-8 as presented.

Turning now to FIG. 10, a more detailed description of the microcomputer130 as implemented in the preferred embodiment is presented. It will benoted that the microcomputer is illustrated as based on an 8086-2microprocessor, of Intel Corp. of Santa Clara, California. Suitablecomputers having the characteristics illustrated by FIG. 10 are commonlyavailable in the market.

Turning now to FIGS. 11-13, a software routine for the microcomputer ofFIG. 10 will be disclosed. With particular reference to FIG. 11, thethree primary software modules and their interactions will be described.

FIG. 11 shows common structural links between modules and how they areinitiated. A shared or common memory 300 is provided to hold variableswhich are used by all modules or units in the subject system. Thisprovides means by which modules which may have been programmed indifferent languages are able to change a state of a stored variableindependently of one another. Each routine is initiated without respectto a status of any other routine. SCCCU operation and signal analysis isoperationally separated from other code modules, but by virtue of theshared memory 300, shares common variables with other code sections.Activation is preferably not user controlled, but instead is a functionof a hardware timer and clock circuit 302 which activates the module ata frequency of suitably 200 times per second, regardless of a state ofthe remaining modules. This section of code is essentially running inthe background while all other code is running in the foreground.Because of its mode of activation, the SCCCU operation and signalanalysis module has priority over other code modules. This module istermed "free-running" and requires no operator interaction to begin orcomplete its task. The software routine implemented in modules 1, 2, and3 (304, 306, and 308 respectively), are illustrated in FIGS. 12, 13, and14, respectively.

The computation of the volume of the right ventricular segments (one isillustrated in FIG. 16) between selected pairs of sense electrodes isdone according to the formula Volume=(I_(c) ×ρ×L²)/V_(EE) where I_(c) isa known constant current source, ρ is the resistivity of the medium, Lis the distance between electrodes and V_(EE) is the measured end to endvoltage. This formula is substantially accurate although it does nottake into consideration the loss of current to surrounding tissue or thevarying conductivity of blood.

If L is designated to be 1 cm., then each segment volume is directlyproportional to ρ/Z_(EE), where Z_(EE) is the impedance of the bloodvolume in the measured segment and is equal to Ic/V_(EE). Using thethermodilution technique, ρ can be determined as well as any signallosses due to the leakage of drive current through surrounding tissue.Thus, the total ventricular volume, i.e. the sum of the segments, can bedetermined by the formula V_(T) =K Σ1/Z_(EE) where K is a constant whichrepresents the effects of blood resistivity and drive signal losses.

It should be appreciated that the potential from each sensing electrodepair can also be used to detect the position of the catheter within thecardiovascular system. The dynamic potentials from the sensing pairs ofelectrodes are examined and the pair location is determined from thetiming and waveform characteristics.

In the present system, the current source for the electrodes is asinusoidal waveform of approximately 20 microamperes at a frequency ofapproximately 2 KHz. Twenty (20) microamperes has been determined to bethe maximum RMS safe current at 2 KHz by the Association for theAdvancement of Medical Instrumentation.

It should also be appreciated that the construction of the inventivesystem allows any electrode to be a sense electrode or a drive electrodeas desired and selected.

This invention has been described with reference to a preferredembodiment, obviously modifications and alterations will occur to othersupon the reading and understanding of this specification. It is intendedthat all such modifications and alterations be included insofar as theycome within the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A diagnostic catheter for use in measuringcardiac output in the right ventricular chamber comprising:(a) anelongated, multi-lumen, flexible member having a distal end and aproximal end, a first lumen extending the entire length of said memberand terminating in a distal port, a second port extending through theside wall of said member at a location immediately proximate of saiddistal end of said flexible member and a second lumen extending fromsaid proximal end of said member to said second port; (b) an expandablesleeve surrounding said member and spanning said second port, saidsleeve inflatable by a fluid introduced into the proximal end of saidsecond lumen; (c) a plurality of ring electrodes secured to the outersurface of said member at predetermined axial spacing including a distalring electrode located a first predetermined distance proximal of saiddistal end of said flexible member, a proximal ring electrode located asecond predetermined distance greater than said first predetermineddistance from said distal end of said flexible member and a furtherplurality of intermediate ring electrodes disposed between said distalring electrode and said proximal ring electrode; (d) a plurality ofelectrical conductors extending longitudinally through a third lumen insaid flexible member from said proximal end of said flexible member andindividually connected to separate ones of said plurality of ringelectrodes; and, (e) a first stiffening member disposed in a fourthlumen in said flexible member and extending from a third predetermineddistance to a fourth predetermined distance proximal of said distal endof said flexible member, said third and fourth distances being greaterthan said second predetermined distance.
 2. The diagnostic catheter asin claim 1 and further including a fifth lumen extending from saidproximal end of said flexible member and terminating in a third portformed through the side wall of said flexible member and located withinsaid first predetermined distance; and a thermistor element exposed toheat conduction through said third port and having conductor meansextending therefrom through said fifth lumen to the proximal end of saidflexible member.
 3. The diagnostic catheter as in claim 2 wherein saidthird port containing said thermistor includes a plastic seal coveringsaid thermistor, said plastic having a thermal conductivity propertyallowing said thermistor to detect a small temperature change rapidly.4. The diagnostic catheter as in claim 1 and further including a fourthport extending through the side wall of said flexible member andcommunicating with said fourth lumen at a location proximal of saidstiffening member.
 5. The diagnostic catheter as in claim 1 and furtherincluding a multi-terminal electrical connector connected to saidplurality of electrical conductors.
 6. The diagnostic catheter as inclaim 1 and further including valve means connected to the proximal endof said second lumen of said flexible member.
 7. A catheter formeasuring cardiac output, comprising:a catheter body having an outerperiphery and a distal section terminating in a distal end and aproximal section terminating in a proximal end; a plurality of spacedelectrodes secured to said body outer periphery along said body distalsection; a plurality of electrical leads, each one extending in saidcatheter body from a respective one of said electrodes to said proximalend of said catheter body; and an elongated rigid means for stiffening aportion of said catheter body, a distal end of said rigid means beinglocated proximally of a proximal-most of one said plurality ofelectrodes, said rigid means being located proximally of said pluralityof electrodes in order to allow them to be spaced away from endocardialtissue when said catheter body is correctly located in a heart.
 8. Thecatheter of claim 7 further comprising a first lumen extendinglongitudinally through said catheter body from said body proximal end toa distal-most one of said plurality of electrodes, wherein saidplurality of electrical leads are located in said first lumen.
 9. Thecatheter of claim 8 further comprising a second lumen which extends froma proximal end of said catheter body to a port located at approximately15 cm from said distal end of said catheter body.
 10. The catheter ofclaim 9 further comprising:a balloon sleeve located adjacent a distaltip of said catheter body; a third lumen communicating said balloon withsaid proximal end of said catheter body; and, a fourth lumen extendingfrom said catheter body proximal end to a port located on said catheterbody distal end.
 11. The catheter of claim 10 further comprising a fifthlumen extending from said body proximal end and terminating in a portformed through side wall of said body and located at a predetermineddistance from a proximalmost one of said plurality of electrodes. 12.The catheter of claim 7 further comprising:a thermistor sensor securedto said catheter body and spaced distally from a distal-most one of saidplurality of electrodes; and, an electrical lead extending in saidcatheter body from said thermistor to said proximal end of said catheterbody.
 13. The catheter of claim 7 wherein said plurality of electrodescomprises ten electrodes which are spaced apart from each other byapproximately 0.8 to 1.0 cm and wherein a distal-most one of saidplurality of electrodes is located at approximately 9 cm from saiddistal end of said catheter body.
 14. The catheter of claim 7 whereinsaid stiffening means comprises an elongated stylet disposed in a lumenof the catheter, said stylet extending proximally from adjacent aproximal-most one of said plurality of electrodes.
 15. The catheter ofclaim 7 further comprising a pair of drive electrodes, wherein a distalelectrode and a proximal electrode of said plurality of electrodes serveas said pair of drive electrodes and wherein said stiffening means solocates said distal electrode and said proximal electrode as to bepositioned adjacent a pulmonic valve and a tricuspid valve of the heart,respectively.
 16. The catheter of claim 7 further comprising a terminalmeans secured to said catheter body proximal end for receiving a freeend of each of said plurality of electrical leads.
 17. A catheter formeasuring cardiac output, comprising:an elongated flexible multi-lumencatheter body having an outer periphery and a distal section terminatingin a distal end and a proximal section terminating in a proximal end; aballoon attached to said distal end of said body; a first lumenextending the entire length of said catheter body and terminating in afirst port which communicates with an interior surface of said balloon;a plurality of spaced electrodes secured to said body outer peripheryalong said body distal section proximal of said balloon; a second lumenextending from a distal most one of said plurality of spaced electrodesto said proximal end of said body; a plurality of electrical leads, eachone extending through said second lumen from a respective one of saidelectrodes to said proximal end of said catheter body; and, a means fortaking blood pressure measurements in a right ventricle of a heart whenthe catheter is fully inserted in the heart, said means comprising athird lumen which extends longitudinally in said catheter body from saidproximal end to a second port intermediate said plurality of spacedelectrodes, wherein said second port is adapted to take blood pressuremeasurements as the port passes through a tricuspid valve and becomesstationary in the right ventricle of the heart.
 18. The catheter ofclaim 17 further comprising:a fourth lumen which extends longitudinallyin said catheter body from said proximal end to a third port which islocated proximally of said plurality of spaced electrodes; and, astiffening member disposed in said fourth lumen distally of said thirdport.
 19. The catheter of claim 17 further comprising a thermistorsensor secured to said catheter body and spaced proximally from saidballoon.
 20. The catheter of claim 17 wherein ten ring electrodes areprovided which are spaced apart from each other by approximately 0.8 to1.0 cm and wherein a distal-most one of said plurality of electrodes islocated at approximately 9 cm from said distal end of said catheterbody.
 21. The catheter of claim 17 further comprising a multi-terminalelectrical connector connected to said plurality of electrical leads.